1. Field of the Invention
The present invention relates to a thermal recording sheet which is superior in dynamic sensitivity, dot reproducibility, image quality, prevention of turning yellow by NO.sub.x, image storage stability and printing aptitude.
2. Description of the Prior Art
In general, thermal recording sheets are normally prepared by individually grinding a colorless or pale colored basic chromogenic dye and an organic color developer such as a phenolic substance to disperse into fine particles, mixing, and further adding a binder, a filler, a sensitivity improver, a slip agent or other additives thereto to obtain a coating color, which is then coated on a substrate such as paper, synthetic paper, plastic films, cloths, and the like. The thermal recording sheet enables color recording by a momentary chemical reaction caused by heating with a thermal pen, a thermal head, a hot stamp, laser light, or the like.
The thermal recording sheets are applied in a variety of areas such as measurement recorders, computer terminal printers, facsimiles, automatic ticket vendors, and bar-code labels, however, with recent diversification and improvement of the recording devices, requirements to the thermal recording sheet have become stricter. For example, with increasing recording speed, it is required to obtain a high-density, sharp color image even with a small heat energy and, in addition, to have improved storage stability in terms of light resistance, weather resistance, and oil resistance.
Furthermore, with the diversification of market needs, demand is increasing for a thermal recording sheet which is superior in dot reproducibility and image quality over the energy regions from low energy to high energy.
At present, these thermal recording sheets have widely spread in medical field such as hospitals, etc. as a measurement paper for electrocardiogram. However, since clinical charts of electrocardiogram are preserved for a long time, there is a trouble with the conventional thermal recording papers which tend to turn yellow in the presence of the trace of NOx gas in the air during the preservation.
In addition, excellent printing aptitude is further required for measurement papers for electrocardiogram as the scales are also printed thereto.